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Compound blade shapes are mainly used for radial diffusers (stator), but are available for centrifugal impellers, too.

Log. Spiral + Straight 2D (axial)

The inlet section of the blades without overlapping is configured as a logarithmic spiral.

The overlapping part is straight. The transition point between these areas can be moved along the logarithmic spiral curve (see stator mean line).

Circular + Free-form 2D (axial)

The inlet section of the blades without overlapping is configured as a circular arc with the boundary conditions radius rLE, blade angle βLE and throat width aLE.

The overlapping part is designed by a Bezier curve with optionally 2 (straight), 3 or 4 Bezier points (selectable by context menu). The transition point between these areas can be moved along the circular arc curve (see stator mean line).  

Calculation of throat width aLE can be done using the conservation of angular momentum (const. swirl) or a specific deceleration ratio alternatively:

a) Constant swirl

Throat width corresponds to the dimensioning in accordance with the conservation of angular momentum (cur)In, whereat the deceleration is increased by using the factor faLE (1.1...1.3).

b) Deceleration

Alternatively one can use the deceleration ratio cLE/cIn (0.7...0.85) for throat width calculation.

 

Trailing edge angle βTE is a result of mean line design for these special blade shapes and therefore cannot be specified explicitly ("var.").